An electrical actuator, that is one using an electrical power in order to operate, could be intended more particularly, although not exclusively, for actuating a control surface of an aircraft, including of a transport airplane, for instance a horizontal rudder, a direction control surface or a lateral control surface of an airplane.
Such an electrical actuator could be an electro-hydrostatic actuator of the EHA (<<Electro-Hydrostatic Actuator>>) type, usually comprising an electronic module, an electric motor, a hydraulic pump, a pressure relief valve, a hydraulic block and a hydraulic jack. Such actuator is controlled by a control current sent to the electronic module. A local slaving system in the electronic module converts this control current into a speed set-point for the electric motor. The latter drives the hydraulic pump using an electric power supplied by the aircraft. The pump then locally generates a hydraulic power for moving the hydraulic jack.
The present invention could also apply to an electrical backup hydraulic actuator, of the EBHA (<<Electrical Backup Hydraulic Actuator>>) type, being a hybrid actuator comprising the characteristics both of a usual hydraulic servo-control and of an electro-hydrostatic actuator of the EHA type. In a nominal situation (failure free), the EBHA actuator operates as a usual servo-control. On the other hand, in the case of a failure affecting the hydraulic mode, this EBHA actuator switches to a power supply and operates as an EHA actuator.
Because of its technology, an EHA actuator is able to locally generate more effort than the maximum level required for its operation. There is thus the risk that the structure on which the EHA actuator is mounted would be submitted to more important efforts than the level for which it has been dimensioned. Thus, in order to protect such a structure, the EHA actuators are generally provided with at least one pressure relief valve limiting the effort generated by the actuator.
It should be noticed, that, in some configurations, the EHA actuator could be caused to operate on the pressure relief valve. In such a case, the motor operates and the hydraulic pump generates a flow circulating in the pressure relief valve instead of supplying the chambers in the jack.
In such a situation, i.e. with a circulation of fluid being maintained between the pump and the pressure relief valve, the EHA actuator could very quickly become damaged, in such an extent that it could no longer be used.
The failure of the actuator could result from two different origins:                an overheat of the fluid. The fluid circulating between the hydraulic pump and the pressure relief valve switches from the opening pressure of the pressure relief valve to the pump return pressure when going thru the pressure relief valve, resulting in overheating the latter very rapidly. Such a heat being stored in the fluid is afterwards dissipated throughout the whole actuator. The resulting increase of the temperature could quickly lead to the actuator becoming damaged; and        the control electronics. When the pressure relief valve is opened and fluid circulates therethrough, the electric motor should supply a high torque for generating the opening of the pressure relief valve, at a speed depending on the slaving error that could thus reach the maximum speed. In order to maintain such a high torque, a current, being also high, should circulate in the electronic module of the actuator. As the electronics of the actuator is not dimensioned for bearing such a level of current permanently, the latter could become damaged very quickly.        
From FR-2,946,401 of the Applicant, an electrical actuator is known, allowing the above mentioned drawbacks to be overcome. Such an actuator is of the type comprising a control module, an electric motor driving a pump according to a set-point value received from said control module, a hydraulic jack comprising two chambers able to be supplied by the pump and generating the effort of the actuator, as well as at least one pressure relief valve for limiting the effort generated by the actuator. According to FR-2,946,401, such an actuator further comprises means for measuring the difference of pressure existing between the two chambers of the hydraulic jack, means for calculating a corrective parameter by means of this difference of pressure, and means for calculating, from said corrective parameter, an auxiliary set-point value allowing, when being applied to the electric motor, to limit the difference of pressure between the two chambers of the hydraulic jack to a limit pressure being lower than an opening pressure of the pressure relief valve. Thus, in order to protect the electrical actuator from being damaged as a result of overheat, FR-2,946,401 provides means allowing to avoid, upon its operation, the opening of the pressure relief valve(s) and thereby to prevent overheating.
Although efficient, the solution provided by FR-2,946,401 could not be used if the accuracy of the sensors and the characteristics of the pressure relief valves of the actuators are such that they cannot allow the opening of the pressure relief valves to be prevented, while meeting the performance requirements of the actuators.
The present invention provides an alternative solution enabling to overcome the above mentioned drawbacks.